Spinal discs provide support between adjacent vertebrae in a spinal column. Over time, discs can rupture, degenerate or protrude outside of their normal space as a result of injury, degradation or disease. In such cases, the condition of the disc can be weakened or compromised to the point that the intervertebral space around the disc collapses. Changes in disc shape can cause the spine to lose its normal curvature, create impingement of nerves in the disc space, and result in chronic back pain.
A number of surgical procedures can be performed to treat damaged or degenerated discs. In one procedure, the disc is removed, and the remaining adjacent vertebrae are fused together by graft material. Graft material may be contained in an interbody implant or cage. An elongated fixation plate is then placed over the segment or segments being fused. The fixation plate includes openings to receive bone screws. Bone screws are driven through the openings and into vertebrae surrounding the segment or segments to secure the plate over the fused segments. Once secured, the plate assists in controlling relative movement of vertebrae as the vertebrae are fused together.
It is common for vertebrae to settle and compress together at segments being fused. In some approaches, a certain amount of settling and compression is desirable to maintain the graft material under constant load to promote bone growth and stronger fusion between vertebrae. Therefore, a number of plate constructs allow the bone screws to translate relative to the plate to allow the vertebrae to settle. For example, U.S. Pat. No. 6,533,786 to Needham discloses a three-level plate having a pair of circular holes at one end of the plate, and multiple pairs of elongated slots along the rest of the plate. The circular holes and slots are adapted to receive bone screws. Screw translation is regulated as a function of the shape and size of the holes or slots. Once the plate is anchored to vertebrae, the circular holes prevent any bone screw translation of the plate at the one end of the plate, fixing the one end of the plate to the underlying vertebral body. The elongated slots permit limited translation of bone screws at other levels, allowing settlement at those segments. Because one end of the plate is fixed, the levels away from the fixed end settle in one direction toward the fixed end of the plate.
U.S. Pat. No. 6,755,833 to Paul et al. shows a plate arrangement having circular holes and elongated slots extending in pairs along the length of the plate, and a flexible band that extends over the holes and slots to prevent backout of the screws after they are driven into bone. As with the Needham plate, screw translation is a function of the shapes and sizes of the holes and slots. Holes and slots with relatively short lengths serve as support apertures to fix the plate to one or more vertebrae. Slots with greater lengths allow the plate to translate at other levels. Starting at one end of the plate and moving to the other end, the length of each slot gets progressively longer, allowing more and more settlement as the slots get longer.
U.S. Pub. No. 2005/0049595 discloses a number of plates with different combinations of circular holes, elongated slots and carriage blocks. The circular holes rigidly fix the plate to an underlying vertebral body, and prevent screw translation relative to the plate at those hole locations. The slots and carriage blocks provide limited translation of screws relative to the plate. Settlement of vertebrae is limited to the specific arrangement of circular holes, slots and carriage blocks. The degree of translation provided by the carriage blocks is controlled by stop surfaces at specific locations on the plate. The amount of translation may be adjusted by using interchangeable carriage blocks of different shapes and styles.
One of the drawbacks of known plating systems is that the predefined hole and slot configurations dictate how settlement will occur, leaving the surgeon very few options for controlling the settlement of vertebrae prior to bony fusion (arthrodesis). Controlling where settlement occurs, and the manner in which settlement occurs, is important because settlement of one segment can seriously impact the function and condition of adjacent segments. In many fusion procedures, the patient has an adjacent disc that does not require surgery, but shows early signs of a degenerative condition. In such cases, it is desirable to prevent the segment adjacent the degenerating disc from settling or subsiding. Any vertebral motion adjacent the degenerating disc can change the spacing around the degenerating disc, which can aggravate the disc, limit its functionality, and accelerate degeneration of the disc. Holes and slots that dictate how settling occurs may not be designed to control settlement where that control is needed.
To avoid impacting adjacent discs, some surgeons choose plates with circular screw holes that rigidly fix the plate and screws, and prevent screw translation and settlement. This has its own set of potential problems. If the screws cannot translate in the plate, and the plate does not allow settlement, the fused segments will be stress-shielded by the screws and plate. The graft material will not be maintained under constant load, limiting bone growth and fusion in the stress-shielded segments.
A related drawback to known plating systems is that many plates limit settlement and screw translation to one direction, toward one end of the plate. For example, a number of plates feature circular holes at one end of the plate and slots arranged along the rest of the plate. The circular holes at the one end of the plate allow no screw translation, fixing the plate to the vertebra at that end. The slots allow their respective screws to translate, permitting settling to occur away from the fixed end. The slots offer no control over settlement at specific levels, however. Therefore, if a surgeon wants to allow settlement to occur at levels located away from the fixed end, but wants to vary the amount of settlement at different segments, the slots provide no control mechanism to do this.
Because slot configurations dictate how and where screw translation occurs, a surgeon may be compelled to carry an assortment of different plates, with each plate kept on hand to address a specific indication. For example, the surgeon may have to obtain a plate with circular holes at a center section or sections of the plate, and elongated slots or carriage blocks at the ends of the plate, to fix the screws at the center section or sections of the plate. For fusion procedures involving three-level plates having four rows of screws, the surgeon may need a plate with circular holes in the two interior rows to fix the plate at those sections, or a plate with circular holes at only one of the interior rows to fix the plate at that section only. Given so many plate options, the process of determining the proper plate to use can be difficult, particularly in procedures involving three or more levels. The surgeon must select a plate having the right combination of holes and slots, and the right selection of slot sizes and shapes, to control settlement in a desired manner.
In summary, systems that control screw translation as a function of slot geometry, carriage blocks and/or stop surfaces provide limited options for surgeons, and may compel hospitals to supply a large number of different fixation plates to give surgeons the options they need to address different contingencies.